The present invention relates to optical switch elements and optical switch matrices and, more particularly, to a method and architecture to realize wavelength-selective switch elements and wavelength-selective switch matrices (“Lambda switches”) on electro-optical substrates such III-V compounds, particularly based on InP and GaAs, LiNbO3, and LiTaO3 using preferably Planar Lightwave Circuit (PLC) technology.
Integrated optical switches are well known. For a recent review of the art using LiNbO3 substrates, see H. Nakajima, “Development on guided-wave switch arrays.” IECE Trans. Commun. Vol. E-82B, pp. 349-356, 1999. Waveguides are created in the electro-optical substrate material by processing the substrate locally to increase the index of refraction. For example, the index of refraction of lithium niobate may be increased locally by diffusing titanium into the substrate. To divert light from one waveguide to another, the waveguides are coupled by local electro-optical manipulation of their indices of refraction. Well-known examples of electro-optical switches include directional couplers, Mach-Zender Interference switches, BOA couplers, digital optical switches and x-switches. Depending on the voltage applied to such a switch, light is thus partly or completely diverted from an input waveguide to an output waveguide.
By appropriately combining waveguides and switches, a switch matrix is formed to switch light from a plurality of input waveguides among a plurality of output waveguides. A variety of switch matrix geometries are known. FIG. 1 is a conceptual illustration of a switch matrix of one such geometry: crossbar geometry. A set of input waveguides 10 crosses a set of output waveguides 12. At the crossing points, the waveguides are coupled by 2×2 switches 14. For simplicity, only three input waveguides 10 and three output waveguides 12 are shown in FIG. 1. Typically the numbers of input waveguides 10 and output waveguides 12 are equal powers of 2, up to a practical maximum of 32.
All the electro-optical switch elements mentioned above have a broad wavelength (frequency) response (compared to the 50-400 GHZ standard spacing in typical WDM systems), i.e. are not wavelength-selective. Wavelength-selective integrated optical switch elements have also been developed for WDM communication systems using integrated acousto-optical LiNbO3 devices. A summary of such devices can be found in “Wavelength-selective devices” by M. K. Smit, A. M. J. Koonen, H. Herrmann and W. Sohler in N. Grote, H. Venghaus (eds.), Devices in Optical Communication Systems, Springer Verlag, Berlin (2000) [hereinafter SMI00] and references therein. Switch matrices with no wavelength selectivity have very high loss in connections of “many inputs to one or many outputs”, since combining multiple inputs to the same output has a high loss penalty. In practice such switch matrices enable only point-to-point or point-to-multi-point (multicasting). However, wavelength-selective switch elements can be used to construct switch matrices that can connect many-to-many points, i.e. many inputs to one or to many outputs. This is being done by using the wavelength dimension as an extra degree of freedom to route signals. One output may be simultaneously connected to multiple inputs, where each input is transmitting signals at different wavelength to that output, thus avoiding signal contention.
The basic building blocks for existing acousto-optical switch elements are polarization splitters and acousto-optical polarization converters. Polarization splitters and electro-optical polarization converters implemented in PLCs are known in the art. They are the basic building blocks for the wavelength-selective switch elements and wavelength-selective switch matrices of the present invention. Their principle of operation is given here: polarization converters in PLCs are normally implemented in LiNbO3 or in III-IV semiconductors, while polarization splitters are also implemented on Silica substrates. A LiNbO3 electro-optic polarization converter consists of a LiNbO3 substrate with a Titanium diffused strip waveguide and finger electrodes of period A [R. C. Alferness, “Elecrooptic guided wave device for general polarization transformations”, IEEE J. Quantum Electron., vol. QE-17 pp. 2225-2227, 1981, and R. C. Alferness and L. L. Buhl, “New low-loss elecrooptic polarization controller for λ=1.32 μm”, in Proc. 4th Int.Conf. Integrated Opt. Optical Fiber Commun. (Tokyo, Japan) 1983 pp. 38-39]. With proper crystal and electrode orientations, it utilizes an off-diagonal element of the electro-optic tensor to achieve coupling between the otherwise orthogonal TE and TM modes. Electro-optic polarization conversion is characterized analytically by the well-known co-directional coupled wave equations. The conversion efficiency is given by:                               η                      TE            ↔            TM                          =                                            sin              2                        ⁢                          {                              k                ⁢                                                                   ⁢                                                      L                    ⁡                                          [                                              1                        +                                                                              (                                                          δ                              /                              k                                                        )                                                    2                                                                    ]                                                                            1                    /                    2                                                              }                                            1            +                                          (                                  δ                  /                  k                                )                            2                                                          (        1        )            Where δ=Δβ/2, Δβ is the effective phase mismatch defined in Eq. (3), L is the interaction length and k is the coupling coefficient. Since LiNbO3 is strongly birefringent, polarization phase matching between the TE and TM modes is necessary. Efficient coupling between the non-synchronous TE and TM modes is achieved by using an electrode period A that satisfies the phase matching condition:(2π/λ0)|NTE−NTM|=2π/Λ  (2)where NTE, and NTM are the effective waveguide indices for the TE and TM mode respectively. This phase matching requirement results in a strong wavelength dependence of the polarization conversion, because for fixed A Eq.2 is satisfied exactly only for λ0. The effective phase mismatch for any other wavelength λ=λ0+Δλ is:Δβ=−(2π/Λ)Δλ/λ  (3)The normalized filter bandwidth (FWHM) can be found from Eqs. 1, 2 and 3 (with kL=π/2):ΔλBW/λ0˜Λ/L=1/N  (4)where N is the number of electrode fingers. Typical devices have bandwidths of ˜1-5 nm.
FIG. 2A shows schematically a wavelength-selective polarization converter. It leaves the state of polarization of the incoming wave as is when it is in its “OFF” state. In the “ON” state it converts an input TE (TM) wave at wavelength λ1 to a TM (TE) wave on the output. Other wavelengths are not affected.
A polarization splitter separates the TE and TM components of an incoming wave, and is shown schematically in FIG. 2B. Depending on the waveguide design of the splitter, it can either “bar” the TM polarization component and “cross” the TE component, or “bar” the TE polarization component and “cross” the TM component. Two concepts have been used to realize polarization splitters on LiNbO3. Both concepts yield polarization splitters with splitting ratios exceeding 20 dB.
The first concept, shown in FIG. 2C, uses a passive directional coupler structure fabricated by applying solely the Ti-indiffussion technique [see e.g. A. Neyer, “Low cross-talk passive polarization splitters using Ti:LiNbO3 waveguide crossings”, Appl. Phys. Lett., vol. 55 pp. 927-929, 1989; F. Tian, Ch. Harizi, H. Herrmann, V. Reimann, R. Ricken, U. Rust, W. Sohler, F. Wehrmann and S. Westenhofer, “Polarization independent integrated optical, acoustically tunable double stage wavelength filter in LiNbO3”, J. Lightwave Technol., vol. 12 pp. 1192-1197, 1994; and F. Wehrmann, Ch. Harizi, H. Herrmann, U. Rust, W. Sohler and S. Westenhofer, “Integrated optical, wavelength-selective, acoustically tunable 2×2 switches (add-drop multiplexers) in LiNbO3”, IEEE J. Selected topics in Quantum Electronics, vol. 2 pp. 263-269, 1996]. The coupler is designed to route TE-polarized waves to the cross-state output, and TM-polarized waves to the bar state output or vice versa, by taking advantage of the polarization dependent refractive index profiles.
The second concept is based on a hybrid Ti-indiffussion/proton exchange technology [N. Goto and G. L. Yip, “A TE-TM mode splitter in LiNbO3 by proton exchange and Ti diffusion”, J. Lightwave Technol., vol. 7 pp. 1567-1574, 1989.]. In the proton exchanged regions the extraordinary refractive index is increased whereas the ordinary is reduced. A 1×2 polarization splitter (Y junction) can be fabricated using Ti-diffused waveguide to carry the TM (in X-cut LiNbO3) polarized waves and a proton exchange branching section to extract the TE polarized waves. This splitter operates by adiabatic evolution of the fundamental mode into that output waveguide which has the highest effective index for a given polarization. 2×2 polarization splitters are produced by incorporating four such adiabatic Y junctions [J. E. Baran and D. A. Smith, “Adiabatic 2×2 Polarization splitter in LiNbO3”, IEEE Phot. Technol. Lett., vol. 4 pp. 39-40, 1992].
Existing acousto-optical switch elements based on polarization splitters and acousto-optical polarization converters have a typical response time of several microseconds, which constitutes a major disadvantage in various uses. They also have a typical size of a few tens of millimeters, which limits integration of such devices to a practical maximum of a few devices at most on a single substrate. Acousto-optic polarization converters and switches are also accompanied by an imposed optical frequency shift equal in magnitude to the acoustic frequency, with a sign determined by whether a phonon was absorbed or emitted during the polarization flip.
In view of the above-listed disadvantages of existing acousto-optical wavelength-selective switches, there is a widely recognized need for, and it would be highly advantageous to have, ultra-fast, compact wavelength-selective switches, with no associated optical frequency shift, and switch matrices based on such switches.